1. Field of the Invention
The present invention relates to an ultrasound imaging apparatus used for obtaining, by use of ultrasonic waves, an echogram of the object to be examined.
2. Description of the Related Art
With an ultrasound imaging apparatus for medical diagnostic imaging and by the ultrasonic pulse-echo method, almost real-time images, such as tomograms of soft tissue in a living body and images of bloodstream flowing in a living body, can be displayed on and observed through the monitor. In addition, ultrasound imaging apparatuses are recognized as safer means for medical diagnostic imaging than radiologic diagnostic imaging apparatuses. Such recognition is based on the fact that the ultrasound apparatuses do not require the exposure of the object to radiation, which exposure is inevitable in the case of the radiologic apparatuses. Ultrasound imaging apparatuses are compact and are not expensive. These are reasons for the wide use of ultrasound imaging apparatuses in the field of medicine.
What follows is a description of an imaging operation with a conventional ultrasound imaging apparatus. Waves of ultrasound pulse are transmitted to the object to be examined from an ultrasound probe of the ultrasound imaging apparatus while the probe has an array of piezoelectric bodies. The control of the orientation for the ultrasonic waves transmitted from the array transducer is achieved by adjusting the timing at which each element of the transducer sends waves of ultrasound pulse. Reflection of the ultrasonic pulses thus transmitted occurs in the object to be examined, and echo signals thus produced are received with each of the elements in the array. The echo signals thus received are added while being shifted by an amount of time corresponding to the differences in the distance between the position of the focal point of the reception and the position of each element. The signal from the focal point of the reception is enhanced in this way. A tomogram is obtained by scanning the entire area within the object to be examined for the focal points. Creation of a tomogram in the conventional ultrasonic pulse-echo imaging apparatus is achieved in the following way. Firstly, echoes produced by the reflection at the interface where the acoustic impedance changes are received. Then, signal amplitude is obtained from the received signals through an envelope detection processing. After that, luminance of each pixel displayed on the monitor is modulated in accordance with the signal amplitude obtained by the above processing to form a tomogram.
Incidentally, it is worthwhile to pay attention to the phase of the echo produced by the reflection at the interface where the acoustic impedance changes. The echo produced at the interface where the acoustic impedance is to be increased has the same phase as the transmitted pulse has. In contrast, the echo produced at the interface where the acoustic impedance is to be decreased has a phase that is opposite to the phase of the transmitted pulse. In other words, the sign that an echo has corresponds to the increase or the decrease of the acoustic impedance that is to take place at the interface.
Japanese Patent Application Publication 2004-113364 discloses a conventional technique to detect the signs of echoes for image creation. What follows is a description of the disclosed conventional technique. Refer to the waveform diagram in FIG. 1B. An ultrasound pulse having a center frequency of a fundamental frequency f0 is superposed with another ultrasound pulse having a center frequency of a second harmonic frequency 2f0 to produce still another ultrasound pulse. The ultrasound pulse thus produced is then transmitted, and its reflection at an acoustic-impedance interface within the object to be examined produces an echo wave, which is to be received. FIG. 1B shows both an echo wave produced by the reflection at an interface where the acoustic impedance is to be increased and an echo wave produced by the reflection at an interface where the acoustic impedance is to be decreased. Then, the received echo wave is subjected to a band-pass filtration processing so as to separate an echo signal having a center frequency of the second harmonic frequency 2f0 from another echo signal having a center frequency of the fundamental frequency f0. The echo signal with the fundamental frequency is squared to obtain a reference echo signal having a center frequency of the second harmonic frequency 2f0. Note that this reference echo signal always has a constant sign irrespective of which of the signs the echo has. With the phase of this reference echo signal being as the reference, the second harmonic echo signal having a center frequency of the same second harmonic frequency 2f0 is subjected to a phase sensitive detection processing. In this way, the sign of the echo is detected.
When the ultrasound echo method is brought into practice, the following fact needs to be taken into consideration. In addition to the phase shift that occurs at an acoustic-impedance interface, there is a phase shift that occurs during the propagation of the waves in the medium in which there is no acoustic impedance. This is why the phase of the above-mentioned reference echo has to be employed as the reference. Such a phase shift that occurs during the propagation is explained in the following way. The attenuation in a living body becomes larger as the frequency becomes higher. Accordingly, as the reflected signal comes from a deeper portion of a living body and so is propagated for a longer distance, the higher-frequency component of the signal is more likely to be lost. As a consequence, the center frequency of the reflected signal is shifted to the lower-frequency side by an amount corresponding to the depth of the portion from which the reflected echo comes. Signal processing carried out on the assumption of a constant center frequency despite the fact that the center frequency is actually shifted causes an apparent phase rotation of the reflected signal. With nothing extra being done, there is no way to distinguish a phase rotation caused by the change in the hardness from a phase rotation caused by the shifting of the center frequency. If a living body were a perfectly homogeneous medium, the correction for the shifting of the center frequency would be possible. Such correction in practice, however, is not so easy because both the sound propagation speed and the attenuation coefficient vary from place to place within a living body. The disclosure of Japanese Patent Application Publication 2004-113364 makes a distinction between the two types of phase rotation by employing the phase of the reference echo as the reference, and extracts the phase rotation caused by the change in the hardness.